Plastic product design extrusion

FIGURE 10.3 Die designs for extrusion of a pipe. (Adapted from Miller, E. ed., Plastics Products Design Handbook, Part A, Materials and Components, Marcel Dekker, New York, NY, 1981.)... 

For low density packaging and insulation foam products, complete extrusion lines are available from several specialist plastics machinery manufacturers which may use one long single screw or twin screw extruder or two single screw extruders in tandem. (Figure 12.14). In the tandem process, the second extruder is designed specifically to combine extruder and heat exchanger functions. 

Under this heading of manipulation is included any process whereby the solid plastics material, usually in powder or nib form, is converted to the finished end product. It embraces moulding by compression, transfer, injection or extrusion, blow moulding, rotational moulding, and any process whereby a layer of comminuted plastics material is heat-sintered into a finished unit. In all of them the product designer has to work very closely with the manufacturer not only of the plastics material, but also with the mould maker and to an ever-increasing extent with the maker of the moulding machine. 

Designers and processors to produce products at the lowest cost have unconsciously used the basic concept of the FALLO approach. This approach makes one aware that many steps are involved to be successful, all of which must be coordinated and interrelated. It starts with the design that involves specifying the plastic, and specifying the manufacturing process. The specific process (injection, extrusion, blow molding, thermoforming, and so forth) is an important part of the overall scheme and should not be problematic. 

The major processing methods that process well over 80wt% of all plastics are extrusion, injection molding, and blow molding. These processes as well as a few others use a plasticator to melt plastics. It is a very important component in a melting process with its usual barrel and screw. If factors such as the proper screw design and/or barrel heat profile are not used correctly fabricated products may not meet or maximize their performance and very important not provide for low cost process. 

Faced with all these unknowns, the food materials scientists cannot yet produce predictive and quantified models of materials behaviour in the extruder or of the post-die processes by which products are formed. Instead we concentrate here on the dominant molecular features that must be understood if even qualitative rules of behaviour and design can be stated. Fortunately, most food engineers now recognise the problem but still would like to attain the degree of predictive coupling between molecular formulation, rheology and product properties which plastics extrusion has to hand. 

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